1. Field of the Invention
This invention relates to improvements in operational amplifiers, and more particularly to improvements in operational amplifiers that have wide input/output voltage ranges approaching rail-to-rail swings, and still more particularly to common-mode current handling and power supply rejection ratio techniques in operational amplifiers of this type.
2. Background Information
Recently, increased interest has been directed towards realizing low distortion class-AB operational amplifiers that have a wide range of input and output voltages, approaching the upper and lower supply rail voltages. It is desirable, of course that the amplifier be capable of being realized in a monolithic integrated circuit structure of bipolar or MOS transistor devices.
An example of a typical operational amplifier circuit is shown by Johan H. Huijsing et al., in "Low-Voltage Operational Amplifier with Rail-to-Rail Input and Output Ranges" IEEE Journal of Solid-State Circuits, Vol. SC-20, No. 6, December, 1985, in which an operational amplifier is advanced that can operate with a supply voltage as low as 1.5 volts. The output voltage can reach the supply rail within 150 mV. The circuit has an input stage that provides rail-to-rail common-mode voltage range and an output stage with rail-to-rail output-voltage swing with accurate class-AB control. The circuit, however, requires an intermediate stage to enable the input and output stages to be connected together to produce sufficient voltage gain to enable the circuit to be used as a general purpose operational amplifier. The provision of the third stage not only increases the size and complexity of the circuit, it reduces the circuit bandwidth and speed.
Jeroen Fonderie et al., in "1-V Operational Amplifier with Rail-to-Rail Input and Output Ranges", IEEE Journal of Solid-State Circuits, Vol. 24, No. 6, December, 1989, shows a bipolar operational amplifier with rail-to-rail input and output ranges for low supply voltages. This circuit again requires a large number of components and an intermediate stage between the input and output stages. Being a three-stage design, its composition is relatively complicated, and has a bandwidth limited to approximately 450 kHz in the technology used.